(a) Technical Field
The present disclosure relates to an improved fuel cell stack. More particularly, it relates to an improved fuel cell stack having improved temperature uniformity, which can reduce variation in temperature distribution of whole cells by a simple change in the structure of a coolant inlet manifold and a coolant outlet manifold in the fuel cell stack.
(b) Background Art
A polymer electrolyte membrane fuel cell or a proton exchange membrane fuel cell (PEMFC) is a device that generates electricity with water by an electrochemical reaction between hydrogen and oxygen. The PEMFC has various advantages such as high energy efficiency, high current density, high power density, short start-up time, and rapid response to a load change as compared to the other types of fuel cells.
The configuration of a fuel cell stack will be briefly described below. A membrane-electrode assembly (MEA) is positioned in the center of each unit cell of the fuel cell stack. The MEA comprises a solid polymer electrolyte membrane, through which hydrogen ions (protons) are transported, and an electrode/catalyst layer such as cathode and an anode, in which the electrochemical reaction between hydrogen and oxygen takes place, disposed on each of both sides of the polymer electrolyte membrane.
Moreover, a gas diffusion layer (GDL) and a gasket are sequentially stacked on both sides of the MEA where the cathode and the anode are located. A separator is located on the outside of the GDL. The separator includes flow fields through which reactant gases (such as hydrogen as a fuel and oxygen or air as an oxidant) are supplied and coolant passes.
A plurality of unit cells each having the above-described configuration are stacked, and an end plate for supporting the unit cells is connected to each of both ends thereof. That is, the unit cells are arranged between the end plates, whereby the unit cells and the end plates are fastened to each other to form the fuel cell stack.
The principle of operation of the PEMFC is as follows. Hydrogen as a fuel and oxygen (air) as an oxidant are supplied to the anode and the cathode of the MEA through the flow fields of the separator, respectively. The hydrogen supplied to the anode as an oxidizing electrode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions are transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane. The electrons are transmitted to the cathode through the GDL functioning as a conductor, the separator, and an external conducing wire. The flow of electrons through the external conducting wire generates electricity.
At the cathode as a reducing electrode, the hydrogen ions supplied through the (polymer) electrolyte membrane and the electrons transmitted through the separator react with the oxygen-containing air supplied to the cathode to produce heat and water.
Since the voltage of each unit cell is low (typically below 1 V) during operation, several tens to several hundreds of unit cells are stacked in series to increase the total voltage produced, thereby forming a fuel cell stack for use as a power generator.
The polymer electrolyte membrane fuel cell exhibits high performance in a temperature range from room temperature to 80° C. As the temperature is lowered, the performance may be reduced by a reduction in reaction activity and a reduction in ion conductivity of the electrolyte membrane.
Temperature considerations are especially important in view of cold weather seasons. For example, when the temperature of the fuel cell stack mounted in the vehicle is lowered below the freezing point as the outside temperature is below zero in winter, the activity of the electrode is reduced. Further, the conductivity fuel cell stack is also reduced because the water that transfers hydrogen ions in the electrolyte membrane is frozen, thereby reducing the performance.
Moreover, when the temperature is low while humidified gas is supplied, a flooding problem occurs due to condensation of water, which has a critical effect on the performance and durability of the fuel cell stack.
Therefore, in order to operate the fuel cell stack in which several hundreds of unit cells are stacked together at an optimal temperature in any environment, it is important to uniformly maintain the temperature distribution of the entire fuel cell stack in a predetermined range.
The end plate including a current collector for maintaining the fastening force and collecting current is located at both ends of the fuel cell stack. It can be seen from numerous tests and papers that the temperature of cells adjacent to the end plate (or current collector) is lower than that of the other cells.
FIG. 1 is a diagram showing the flow of coolant in a conventional fuel cell stack, in which a separator for separating unit cells 11 of a fuel cell stack 10 is stacked, a coolant inlet manifold 13 is provided at one side of the fuel cell stack 10, and a coolant outlet manifold 15 is provided at the other side of the fuel cell stack 10 such that the coolant supplied to the coolant inlet manifold 13 passes through the unit cells 11 and the resulting coolant is collected in the coolant outlet manifold 15 at the opposite side and then discharged to the outside.
That is, the coolant introduced into the coolant inlet manifold 13 is distributed to each coolant channel 11b formed in the separator of the cell 11 to cool the corresponding cell 11 and is then collected in the coolant outlet manifold 15. The coolant collected in the coolant outlet manifold 15 is finally discharged to the outside of the fuel cell stack 10.
As such, when the coolant moves from the coolant inlet manifold 13 to the coolant outlet manifold 15 through the cells 11, it absorbs heat generated by the electrochemical reaction in the fuel cell stack 10, thereby cooling the fuel cell stack 10.
As the coolant collected in the coolant outlet manifold 15 absorbs heat during the cooling process, the temperature of the coolant is higher than that of the coolant initially supplied to the coolant inlet manifold 13.
In the conventional fuel cell stack, the temperature of the cells adjacent to the end plate 19 and the current collector 18 is relatively low during initial start-up.
FIG. 2 shows the temperature distribution of the coolant in the coolant outlet manifold of a conventional fuel cell stack, from which it can be seen that the temperature is lowered as it goes to the downstream side with respect to the coolant flow direction.
FIG. 3 shows the analysis results of a one dimensional heat transfer model during cold start-up of the conventional fuel cell stack, from which it can also be seen that the temperature of the cells more adjacent to the end plate is lower during cold start-up. Therefore, it is necessary to improve the temperature distribution in the fuel cell stack by uniformly maintaining the temperature of the whole cells.
Conventionally, a thick device for thermally insulating or heating is inserted between the end plate and the stacked cells to prevent temperature reduction.
For example, U.S. Pat. No. 6,824,901 discloses a method of inserting a thick insulator between an end plate and a separator to thermally insulate the region where a reaction occurs. The patent also discloses disposing a plane heater between the end plate and the separator to maintain the temperature of the entire fuel cell stack at a predetermined level.
In another example, Korean Patent No. 747,865 (Aug. 2, 2007) discloses a fuel cell stack in which a current collector is formed of at least one material having a different coefficient of thermal expansion to use a change in thickness according to the temperature, i.e., a difference in contact resistance according to the temperature. That is, when the temperature is low, the contact resistance is increased by the contraction of the material having a high coefficient of thermal expansion such that the current collector serves as a heater by the resistance as well as the current collection. When the temperature is low, the contact resistance is reduced such that the current collector performs only the current collection.
In yet another example, Korean Patent No. 747,869 (Aug. 2, 2007) discloses a stack fixture structure for cold start-up of a fuel cell vehicle, in which a cover for covering the outside of an end plate is attached to a fuel cell stack to enable the cold start-up of a fuel cell, thereby forming an air layer for thermal insulation.
However, these prior art solutions have significant disadvantages. In the case where the entire end plate is thermally insulated, the thickness of the insulator for the thermal insulation should be increased, which increases the thickness of the entire fuel cell stack. In the case where the cover is attached to the outside of the end plate, it is impossible to prevent the heat generated in the electrode from being transferred to the end plate.
Moreover, in the case where the heater is disposed between the end plate and the separator, it is necessary to supply power for the operation of the heater from the outside, and thus an auxiliary device for power supply should be provided. Moreover, since the operation of the heater should be controlled, the control system is complicated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.